Waste water treatment plants generally separate solids from the liquid and generally consist of two basic stages: primary treatment and secondary treatment. In the primary treatment stage, larger solids are removed from waste water by settling. Secondary treatment is a large biological process for further removal of the remaining suspended and dissolved solids. Secondary treatment removes up to 85% of the remaining organic material through a biological process and cultivating and adding sewage microorganisms to the waste water. This process is accomplished in a trickling filter or an aeration tank.
Waste water treatment plants generally use aeration tanks to suspend microorganisms in waste water. After leaving the primary treatment stage, sewage is pumped into aeration tanks. The sludge is loaded with microorganisms and mixed with air or pure oxygen. As air is forced into the aeration basins, the air increases the activity of these microorganisms and helps keep the organic waste thoroughly mixed.
Dissolved oxygen (DO) is added to the aeration basin to enhance the oxidation process by providing oxygen to aerobic microorganisms so that they can successfully turn organic wastes into inorganic byproducts. In order to effectively metabolize food and reproduce, each microorganism generally requires at least 0.1 to 0.3 mg/L DO. Most waste water treatment plants maintain about 2 mg/L of DO so that the microorganisms contained in the floc can get sufficient oxygen. A floc is generally a clump of solids formed in sewage by biological or chemical action. If the dissolved oxygen is less than 2 mg/L, the microorganisms in the center of the floc may die since the microorganisms on the outside of the floc use the DO first. If this happens, the floc breaks up. If the DO content is too low, the environment is not stable for these microorganisms and they will die due to anaerobic zones, the sludge will not be properly treated, and the waste water treatment plant will be forced to conduct an expensive and time-consuming biomass replacement process. Because of this risk, many waste water treatment plants compensate by adding excessive amounts of dissolved oxygen to their process. However, when the dissolved oxygen levels become too high, energy is wasted, expensive aeration equipment undergoes unnecessary usage, and unwanted microorganisms (filamentous biology) are prompted.
Power costs associated with the operation of the aeration process and secondary treatment generally run from 30 to 60% of the total electrical power used by a typical waste water treatment facility. Equipping the aeration basin with on-line dissolved oxygen measurement automates the aeration system to maintain the correct amount of dissolved oxygen. Further, waste water treatment plant energy costs may be reduced significantly by using on-line dissolved oxygen measurement.
Dissolved oxygen introduced into the aeration basins also provides the added benefit of mixing, thus bringing the microorganisms, oxygen and nutrients together. Mixing also removes metabolic waste products. Finally, the mixing or aeration keeps this floc suspended and prevents it from settling to the bottom.
Continuous and precise measurement of dissolved oxygen is cost effective, keeps the waste treatment process functioning properly, and eliminates the need for frequent sampling and laboratory sampling.
Various types of dissolved oxygen sensors are known. However, all dissolved oxygen sensors stop working effectively when they become coated with biofilm or a slime layer. Thus, such sensors generally require regular cleaning (weekly or more often) in order to ensure that the dissolved oxygen can be efficiently sensed. In order to easily clean dissolved oxygen sensors, it is known to use pressurized gas and a nozzle pointing at the sensing surface of the dissolved oxygen sensor to remove the biofilm or sludge from the sensing surface. For example, the model DO-03/04 Dissolved Oxygen Measuring System with Air Blast Cleaner, available from Rosemount Analytical, Inc., of Irvine, Calif., provides a complete system including a sensor, analyzer, sensor washer head, mount hardware, and air compressor. This complete system automatically generates air blasts that remove any biofilm, slime or sludge that may have accumulated upon the sensor. However, while this automated system is highly convenient, the sensor may still require periodic calibration in order to ensure that its readings are accurate.
During calibration, a fluid (gas or liquid) with a known dissolved oxygen content is exposed to the dissolved oxygen sensor. Then, the response of the dissolved oxygen sensor to the known quantity is recorded and used to compensate, or otherwise adjust, later-read values of the dissolved oxygen sensor during operation. Field maintenance in waste water treatment facilities is relatively unpleasant, and generally the less one has to be exposed to such substances, the better. Providing a dissolved oxygen measurement system with the ability to reduce technician effort and expense required for periodic calibration would represent a significant benefit to the art.